Recovery of titanium dioxide from titanium oxide bearing materials like steelmaking slags

ABSTRACT

A method of recovering titanium dioxide from a titanium oxide bearing raw material, such as e.g. steel slags rich in TiO2 includes the steps of grinding the titanium dioxide raw material, reacting the particulate raw feed material with sulphuric acid under specified conditions, digesting and filtering the resultant cake material containing titanyl sulphate, hydrolysing the titanyl sulphate and, after washing the hydrolysate, calcining the hydrolysate to produce titanium dioxide.

BACKGROUND OF THE INVENTION

[0001] THIS invention relates to the recovery of titanium from titaniumbearing materials, and in particular to a method of recovering titaniumdioxide or titanium metal from a titanium dioxide bearing material.

[0002] Highveld Steel and Vanadium Corporation is a large manufacturerof steel using its own unique steel manufacturing process. The slagproduced in this steel manufacturing process is rich in titaniumdioxide, typically in amounts of 22 to 32% of the slag material.

[0003] Pure titanium dioxide is white in colour and is, therefore, avaluable pigment used in many applications such as the production ofpaints, paper, cement, polymers and the like. The slag produced in theHighveld Steel manufacturing process is an ideal source of titaniumdioxide for this purpose. However, there is currently no viablecommercial process for recovering the titanium dioxide from the slagmaterial.

SUMMARY OF THE INVENTION

[0004] A method of recovering titanium dioxide from a raw materialcontaining the titanium dioxide comprises the steps of:

[0005] a) grinding the titanium dioxide bearing material to form aparticulate raw feed material;

[0006] b) contacting the particulate raw feed material with apredetermined amount of sulphuric acid in a reaction vessel and raisingthe temperature in the reaction vessel to a predetermined temperature atwhich a reaction takes place to produce a cake material containingtitanyl sulphate;

[0007] c) contacting the cake material with a sufficient quantity ofwater, and optionally recovered process acid, to dissolve the cakematerial, which contains the titanyl sulphate;

[0008] d) filtering the resultant suspension and collecting the solutioncontaining the titanyl sulphate;

[0009] e) hydrolysing the titanyl sulphate containing solution bycontacting the solution with water, which has first been seeded with anappropriate amount of rutile and heated, or a portion of previouslyhydrolysed solution containing hydrated titanium dioxide, and heatingthe solution to boiling point to precipitate out hydrated titaniumdioxide;

[0010] f) washing the hydrolysate with an ammonium solution to removeresidual sulphate as ammonium sulphates followed by filtering off thehydrated titanium dioxide; or

[0011] g) filtering the hydrolysate followed by washing with sodiumhydroxide, ammonium hydroxide, water, phosphoric acid and/or dilutedsulphuric acid; and

[0012] h) calcining the hydrolysate to drive off any residual acid andwater of crystallisation to produce titanium dioxide.

[0013] The slag in step a) is preferably ground to form a particulatematerial in which at least 80% of the particles are able to pass througha 175 micron mesh, more preferably a 45 micron mesh.

[0014] The reaction of the sulphuric acid solution and particulate feedmaterial in step b) typically takes place in a fusion reactor, which maybe a batch or continuous fusion reactor.

[0015] The temperature is preferably raised in the fusion reactor byintroducing pre-heated air into the reaction vessel.

[0016] Cold air is preferably blown through the cake material, inparticular for a period of about 4 hours, after the reaction iscompleted to produce a porous cake.

[0017] The porous cake is preferably left to mature for an appropriatetime, typically about 8 hours.

[0018] In step c), air is preferably introduced with the water, andoptionally recovered process acid, in order to assist with agitation todissolve the cake.

[0019] The air is preferably cold air to control the reactiontemperature, preferably below about 85° C., more preferably below about75° C., in order to prevent premature crystallisation of TiO₂.

[0020] After the desired amount of water has been introduced, air andmechanical agitation is used to break the cake into a homogenoussuspension.

[0021] The hydrolysis step e) is preferably carried out in the absenceof a prior crystallisation and vacuum concentration step having takenplace.

[0022] The slag material typically also contains V₂O₅, FeO and calcium.The calcium is typically removed as calcium sulphate, typically duringthe filtration step d). The vanadium and iron are typically removed asVOSO₄ and FeSO₄ or FeOSO₄ in the solution remaining in step e).

[0023] If necessary, the titanium dioxide produced in step h) may befurther-purified. The additional purification method preferablycomprises the steps of:

[0024] i) bricketing or pelletising the titanium dioxide and subjectingit to a chlorination step, in particular gas chlorination, in order toproduce gaseous TiCl₄ ⁻;

[0025] j) condensing the gaseous TiCl₄ to produce a crude liquid TiCl₄;

[0026] k) distilling the crude liquid TiCl₄ to produce a substantiallypure TiCl₄ liquid; and either

[0027] l) gasifying the pure TiCl₄ liquid to produce gaseous TiCl₄ andoxidising the gaseous TiCl₄ to produce titanium dioxide pigment, or

[0028] m) treating the pure TiCl₄ liquid in a conventional process, suchas a Krohl process, to produce metallic titanium.

BRIEF DESCRIPTION OF THE DRAWING

[0029] The invention will now be described in more detail, by way ofexample only, with reference to the accompanying drawing which is aschematic flow diagram of a preferred embodiment of a method ofrecovering titanium dioxide according to the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0030] The present invention is directed at a so-called sulphatingprocess for recovering titanium, in particular titanium dioxide, from atitanium bearing material, in particular a slag produced in a steelmanufacturing process containing titanium dioxide.

[0031] In the first embodiment of the invention, the titanium bearingmaterial is ground to an appropriate particle size and classified. Forease of describing the process, reference is made to a slag produced ina steel manufacturing process, the slag containing about 22 to 32%titanium dioxide. The raw material is ground into a particulate materialuntil at least 80% of the titanium dioxide in the slag is liberated andis able to pass through a 175 micron mesh, more preferably a 45 micronmesh.

[0032] The particulate material is then reacted with sulphuric acid in acontinuous or batch fusion reactor. In order to carry out the reaction,the desired amount of sulphuric acid, typically a 93% to 96% sulphuricacid solution at suitable temperature, is firstly introduced into thefusion reactor. The particulate feed material is then added to thesulphuric acid solution. Preheated air, or steam if appropriate, is thenintroduced through the bottom of the reactor and allowed to rise throughthe reaction mix in order to heat the reaction mix to the point wherethe reaction commences. The heated air is normally introduced at about400° C. and the reaction usually commences at around 100° C.

[0033] The exothermic reaction of the sulphuric acid with the fine feedmaterial is a violent reaction, which is evident from the white fumes ofSO₂/SO₃ coming off the reaction mixture. After the reaction hasproceeded substantially to completion, a dense cake containing titanylsulphate is formed. The air velocity is increased and allowed to passthrough the cake material in order to “mature it” i.e. to produce aporous cake material.

[0034] Once the porous cake material has been produced, water isintroduced from the bottom of the reactor and allowed to flow throughthe cake to begin digestion thereof. During this leaching step, it isimportant to introduce cool air into the reactor to keep the temperaturebelow about 85° C., preferably below 75° C., depending on the feed stock(e.g. if it has a high chromium content) to avoid prematureprecipitation of titanium dioxide. The leaching process is continueduntil the cake material has been completely digested, resulting in aliquid to solid ratio of about 1:1, with a high concentration of TiOSO₄,VOSO₄, and FeOSO₄ or FeSO₄. Typically, the cool air is allowed to passthrough the cake and act as a mixing agent or agitator until the cakehas been completely digested and a suspension is produced. This stage ofthe process typically takes about 4 hours until completion.

[0035] Once the cake material has been digested, the reactor is drainedand the suspension subjected to a filtration step in an appropriatefilter system (such as a settling drum and press) to remove calciumsulphate and other slag residue. The calcium sulphate and slag residuecan be further treated in a conventional way to recover H₂SO₄ to makethe tailings more environmentally friendly.

[0036] The solution containing TiOSO₄, VOSO₄, FeOSO₄ and FeSO₄ andminute amounts of calcium is a very dense solution (having theappearance of an oil) due to the high concentration thereof.

[0037] The dense solution is processed in a hydrolysis step in order toprecipitate out the TiO₂xH₂O. During this hydrolysis step, about 1%nuclei rutile is added to water at 25° C., whereafter the water isheated to a temperature of about 60° C. The titanyl sulphate solution isthen added to the seeded water and the temperature raised to boilingpoint (+/−95° C.), at which stage hydrolysis takes place. Hydrolysis isclearly evident from the white suspension which is produced. This stepis allowed to proceed for +/−1 hour in order for hydrolysis to becompleted. The hydrolysate is then filtered in a hot filtration step inorder to recover the hydrated titanium dioxide. The filtration iscarried out in a hot filtration step in order to keep the particle sizelarge enough to be captured by the filtering medium.

[0038] The liquor is a misty blue colour due to the Fe and V componentswhich can be recovered from the liquor. Once the Fe and V have beenrecovered, the liquor may be reused for washing or leaching.

[0039] The hydrolysate recovered in the filtration process containsTiOxH₂O.SO₃. This product is washed with an ammonium solution, typicallya 12,5% NH₃ solution, to remove any residual Fe and V and also to removesulphate as ammonium sulphate.

[0040] The resultant TiO₂xH₂O hydrolysate, containing about 91% TiO₂,and some SO₄ ²′, VOSO₄, FeOSO₄ and FeSO₄, is then calcined in anoxidative environment by passing air through the product at atemperature of about 950° C. for about an hour. The SO₃ is driven offand can be recovered as H₂SO₄ together with the waters ofcrystallisation. The resultant titanium dioxide product has a purity ofabout 98%.

[0041] If desired, the leachate containing the VOSO₄ and FeOSO₄/FeSO₄ iscrystallised, and filtered to recover (NH₄)₂SO₄ fertiliser. The solutioncontaining the Fe and V components can then be treated in a conventionalmanner to recover the iron and vanadium.

[0042] As mentioned previously, pure titanium dioxide is a very valuableproduct as a white pigment. However, a 98% titanium dioxide product isnot suitable for this purpose. Accordingly, the titanium dioxiderecovered is further purified by a purification process in order toproduce a product greater than 99.99% titanium dioxide.

[0043] The first step in this additional purification process is a gaschlorination step. In this step, the titanium dioxide is bricketed orpelletised and then introduced into a shaft furnace or salt bath towhich is added a small amount of activated carbon. Chlorine gas is thenpassed through the bottom of the bed at ambient temperature. Thetemperature in the bed typically reaches about 900° C. whereupon gaseousTiCl₄ is driven off.

[0044] The gaseous TiCl₄ is then condensed in a conventional condenserand the crude liquid TiCl₄ containing FeCl₃ and VOCl₃ is recovered.

[0045] The crude TiCl₄ liquid is distilled in a distillation column witha sodium chloride crystals reactor to remove the FeCl₃ and some of theVOCl₃. The remainder of the VOCl₃ can be removed by a fractionaldistillation process with aluminium as a reagent in a conventionalmanner. The VOCl₃ recovered in the fractional distillation process canthen be further treated to recover vanadium.

[0046] The purified TiCl₄ liquid is then oxidised in an oxidationprocess in order to produce titanium dioxide pigment having greater than99.99% purity. Alternatively, the pure TiCl₄ liquid can be treated in aconventional Krohl process to produce titanium metal.

[0047] The above embodiment of the invention will now be described inmore detail with reference to the following non-limiting example.

EXAMPLE

[0048] The slag used in this example comprised the following components:CaO  14% MgO  13% SiO₂  25% Al₂O₃  15% S 0.3% TiO₂  25% V₂O₅ 0.7% FeO6.5%

[0049] The raw slag material was ground and classified until 80% of thefine particulate material had an average diameter of less then 175microns, as determined by passing the particulate material through a 175micron mesh.

[0050] 15 Litres of 93% H₂SO₄ was introduced at 25° C. into a sealedreactor. 20 kg of fine slag material was then introduced into thereactor and after about 2 minutes the temperature rose from about 25° C.to about 63° C., which is indicative of the exothermic reaction alreadytaking place. Air, which had been pre-heated to 400° C., was thenintroduced into the bottom of the reactor. The appropriate valve wasopened to about 25% of its capacity. Within 5 minutes, the temperatureof the reaction mixture had reached 100° C. whereupon the exothermicreaction started to take place, as was evident from the violent whitefumes of SO₃ coming off the reaction mixture. After a further 3 minutesthe temperature had risen to about 173° C. and continued to rise to amaximum of about 210° C., whereafter the temperature started to drop.After a further 10 minutes, the temperature had dropped to about 100° C.at which time cake formation commenced. The air valve was then opened toits full capacity and the temperature once again rose to about 130° C.whereafter it dropped off. The air was allowed to continue flowingthrough the cake mass for a period of 6 hours to form a porous cake.

[0051] The porous cake was then leached by passing water from the bottomof the reactor together with compressed air to keep the temperaturebelow about 90° C. It is important to keep the temperature below 90° C.,preferably below 85° C., more preferably below 75° C., in order toprevent premature hydrolysis of the titanium dioxide. The liquid tosolid ratio was found to be 1:1 with a high concentration of TiOSO₄,VOSO₄, FeSO₄ and FeSO₄. The air was allowed to continue bubbling throughthe mixture for 4 hours, acting as a mixing agent and agitator, untilthe entire cake had been digested to produce a liquid suspension. Thereactor was then drained.

[0052] The suspension was filtered to remove calcium sulphate and slagresidue. The leachate or solution containing the titanyl sulphate,VOSO₄, FeOSO₄ and FeSO₄ and ppm amounts of calcium, which was a verydense solution (almost oil like) due to its high concentration, was thenhydrolysed to obtain the titanium dioxide. In order for hydrolysis totake place, an appropriate amount of water was seeded with about 1%nuclei rutile and then heated to 60° C. The titanyl sulphate solutionwas then added to the preheated seeded water solution and thetemperature raised to boiling point (+/−95° C.) whereafter hydrolysistook place, producing a white suspension. The process was continued foran hour in order for hydrolysis to be completed, whereafter the hotsuspension was filtered to recover the hydrated titanium dioxide. Thishydrolysate was then washed with a 12,5% NH₃ solution to remove anyresidual Fe and V and also residual sulphate as ammonium sulphate. Thefiltered hydrolysate was removed and calcined in an oxidativeenvironment at 950° C. for 1 hour in order to drive off SO₃ and watersof crystallisation.

[0053] The resultant product was a white titanium dioxide which wasanalysed and found to be +/−98% pure.

[0054] Although the 98% titanium dioxide was not further purified, it isbelieved that the additional purification process described above can beused to produce greater than 99.99% titanium dioxide from the 98%titanium dioxide material.

[0055] Although the additional purification step results in a greaterthan 99.99% titanium dioxide product, chlorine gas is an extremelydangerous product and has to be used under strictly controlledconditions. The applicant therefore set about adapting the process toproduce 99.9% titanium dioxide without the need for an additional gaschlorination stage. It was surprisingly found that by optimising theprocess steps in a number of areas, a titanium dioxide product having apurity of greater than 99.9% can be obtained.

[0056] Accordingly a second, particularly preferred embodiment will nowbe described with reference to the accompanying drawing.

[0057] The titanium slag available is dried as needed and ground to auniform fine particle size. The pulverized slag is mixed withapproximately 1.3 parts by weight of 93% sulphuric acid and heated withpre-heated air in a reactor vessel. At about 100° C. the exothermicreaction starts and the slag is converted into a solid mass composed ofsoluble titanium, vanadium and iron sulphates. The use of heated airinstead of heated steam has been found to be preferable. The reason forthis is that the Highveld Steel slag used in the process has free ironpresent, in an amount of about 4%, in the ferrous state. Accordingly, itis undesirable to have water in the fusion reaction as it wouldtransform the free iron to the Fe⁺⁺⁺ state. In addition, the presence ofoxygen in the heated air assists in the recovery of the titanium byconverting Ti⁺⁺⁺ to Ti⁺⁺⁺⁺.

[0058] Cold air is then blown through the cake for about 4 hours afterthe reaction is completed to produce a porous cake. The cake so producedis left to mature for about 8 hours.

[0059] The reaction cake is then digested in water, and optionallyrecovered process acid, in a ratio of water to solid of about 1.5:1 inorder to dissolve the desired titanium compounds. During theintroduction of water, air is introduced simultaneously to assist withagitation, as mechanical agitation would not at this stage be effectivedue to the solid cake formed. During the introduction of air and wateran exothermic reaction takes place. As it is important for the reactiontemperature to be controlled below 75° C., in order to avoid prematurecrystallisation of the TiO₂, proper flow control of the cold air intothe reactor vessel is maintained. After the desired amount of water hasbeen introduced, air and mechanical agitation is used to break the cakeinto a homogeneous suspension. The appropriate time to achieve thesuspension is in the order of 4 hours, but visible inspection willindicate when this has been achieved.

[0060] Some of the titanium compounds which are believed to go intosolution when digesting the cake with water include:

TiO₂ .xH₂O

TiOSO₄H₂O

TiOSO₄.2H₂O

TiSO₄.H₂SO₄.2H₂O

TiSO₄.H₂SO₄.H₂O

Ti(SO₄)₂.

[0061] Where ilmenite is used as a raw material, the solution typicallycontains trivalent or “ferric” iron. This is reduced to the divalent or“ferrous” form with scrap iron as reducing agent. This step is notrequired in the case of the Highveld Steel slag as the iron in the slagis already in the “ferrous” state.

[0062] The suspension so obtained is filtered through a filter system,typically using large settling tanks. The filtrate consists mainly ofcalcium sulphate CaSO₄ and residue. The CaSO₄ residue can be treated torecover H₂SO₄. A very dense yellow solution is obtained after filtering,which is rich in peroxide TiO₃.2H₂O. By way of information, the peroxideis an oxidising substance which is unstable in the presence of water andis much more soluble than TiO₂ or TiO₂.H₂O. It dissolves in acidsolutions with the formation of yellow to red pertitanyl ions (TiO₂′).In alkaline solutions, it forms titanate ions (HTiO₃—) and/or colourlesspertitanate ions (HTiO₄ ⁻ and TiO₄ ⁻). The solubility of the peroxide isaround 1 g. mol/l at a pH of 0.5 (acid medium) and at a pH of 12(alkaline medium). By the action of hydrogen peroxide on very acidsolutions of tri- or tetravalent titanium, a solution of peroxidizedTiO₂ ⁺⁺ ions is obtained, which deposits as a precipitate of peroxide,TiO₃.2H₂O, upon increasing the pH.

[0063] During the hydrolysis step, the titanium solution is transformedinto a white titanium oxyhydrate slurry. The steps that have gone beforeare fundamental in preparing the titanium compounds for hydrolysis. Onestep which is not required when using Highveld slag is crystallizationand vacuum concentration. Thus, hydrolysis is carried out by contactingthe titanyl sulphate containing solution with heated water which hasbeen seeded with nucleating or seeding agents, in particular nucleirutile, and then boiled.

[0064] The formation of titanyl hydroxide proceeds according to thefollowing reactions:

[0065] To increase the rate of thermal hydrolysis of sulphate solutionsat atmospheric pressure and at the same time obtain products of pigmentgrade, nucleating or seeding agents are added. Normally only 1% nucleior seed agent is required. The composition, purity, and physicalproperties of hydrolytically precipitated titanium dioxide depend to alarge extent upon the conditions under which the decomposition takesplace, such as composition of the solution employed, temperature, andduration of boiling. In the commonly employed processes, large changesin the concentration of the solution would take place as the hydrolysateis formed and an equivalent amount of acid is liberated. Thus theformation of titanic acid will take place under entirely differentconditions at the beginning and at the end of the operation.

[0066] To overcome this effect, a titanium-rich solution is preparedfrom the slag, transferred into a precipitation vessel and heated untilpractically complete hydrolysis has taken place. Four fifths of theliquor is then removed. To the remaining one fifth, still at theprecipitation temperature, fresh pregnant solution is added at such arate as to secure a practically constant concentration of dissolvedtitanium until the vessel is filled. Heating is continued throughout theprocess. The supply of solution is then interrupted, and four fifths ofthe liquor is again removed. The operation is repeated as often as isnecessary. The above process only requires initial introduction ofnuclei or seeding agents, thereafter the one fifth liquor containsenough nuclei seeding agents to initiate the hydrolysis reaction. Thetotal cycle time of the exercise is between 3 and 6 hours.

[0067] The TiO₂.xH₂O is removed by means of filtering through afiltering system. The hydrolysate is then washed with either sodiumhydroxide, ammonium hydroxide, water, phosphoric acid or dilutesulphuric acid to improve the properties of the titanium white.Contaminants that would impart undesirable colour to the finishedproduct are removed from the hydrolysate by extensive washing on specialfilters.

[0068] The iron/vanadium solution can be heated with a 25% NH₃ solution,crystallised and filtered to recover the iron and vanadium and(NH₄)₂SO₄.

[0069] The main reason for washing the hydrolysate is to neutralize theliquor and to improve the crystal properties. Conditioning agents suchas dilute acids and zinc or aluminium powder or a powerful non-metallicreducing agent or phosphoric acid or an alkaline metal could also beintroduced during this washing stage, to ensure the formation of therutile structure during the calcination process that follows.

[0070] The thoroughly purified and washed hydrolysate obtained by thethermal hydrolysis of titanium salt solutions is an amorphous hydrousoxide which still contains impurities as chemi-adsorbed acid. Inaddition, it is too fine-grained and almost amorphous, which isundesirable for pigment grade TiO₂. In the production of pigment gradeTiO₂, accordingly, a calcination step is necessary to drive off thewater and residual acid and at the same time convert the titaniumdioxide to the crystalline form of a required particle size. At the sametime, desired pigmentary properties are developed.

[0071] Amorphous titanic oxide or hydroxide (TiO.xH₂O.SO₃), (such as isobtained from the sulphate solution, is converted to the cryptocrystalline modification of pigment grade TiO₂ by calcination at 950° C.for 1 hour.

[0072] Grinding, classifying and pulverizing of the calcined productproduces the TiO₂ pigment material having a purity of greater than99.9%.

[0073] The various solutions produced in a test process of the inventionwere analysed by Anglo American Research Laboratories (Pty) Ltd for thepresence of Si, Ca, Ti, V and Fe, and the results thereof are set out intable 1 below. TABLE 1 Analysis of Solutions by ICP-OES Si Ca Ti V Femg/L mg/L mg/L mg/L mg/L 1 11 209 60498 2025 20952 2 4.4 98 8632 4754943 3 51 178 18 186 4 14 354 12887 777 11914 5 80 1045 74 1151 8 1514922 267 4077

[0074] Key to the above table:

[0075] 1. Solution obtained during the 1:1 water digestion of the fusedcake in step 2.

[0076] 2. The filtrate solution obtained after the hydrolysis in step 4.

[0077] 3. The filtrate wash solution obtained in the hydrolysis step 4when the TiO₂.xH₂O is removed by filtering and then washed.

[0078] 4. The filtered solution after hydrolysis (batch 1).

[0079] 5. The filtrate wash solution (batch 1).

[0080] 8. The filtrate solution after hydrolysis (batch 2).

[0081] Various batches of TiO₂ obtained in the abovementioned processwere analysed to determine the optimum calcination parameters. Theresults of this analysis is set out in table 2 which follows. TABLE 2Analysis of TiO₂ Fe S V TiO₂ % μg/g μg/g μg/g 6 99.9 413 1969 176 7 97.4234 18817 26 9 99.9 525 446 198 10 81.3 546 65126 205

[0082] From the above table, it is evident that at least 1 hour isrequired for obtaining 99.9% TiO₂. However, in order to reduce theamount of S (in the form of SO₃), 2 hours in the calcining operation wasfound to be optimum.

[0083] From the above results, it is evident that it is possible toobtain pigment grade the titanium dioxide having a purity of 99.9% orgreater without having to carry out an additional purification method.

[0084] As the process is able to produce titanium dioxide which isextremely pure, it provides an ideal means of recovering titaniumdioxide from slag or other titanium dioxide bearing materials to producewhite pigment or titanium metal, valuable products in industry.

1. A method of recovering titanium dioxide from a raw materialcontaining the titanium dioxide comprising the steps of: a) grinding thetitanium dioxide bearing material to form a particulate raw feedmaterial; b) contacting the particulate raw feed material with apredetermined amount of sulphuric acid in a reaction vessel and raisingthe temperature in the reaction vessel to a predetermined temperature atwhich a reaction takes place to produce a cake material containingtitanyl sulphate; c) contacting the cake material with a sufficientquantity of water, and optionally recovered process acid, to dissolvethe cake material, which contains the titanyl sulphate; d) filtering theresultant suspension and collecting the solution containing the titanylsulphate; e) hydrolysing the titanyl sulphate containing solution bycontacting the solution with water, which has first been seeded with anappropriate amount of rutile and heated, or a portion of previouslyhydrolysed solution containing hydrated titanium dioxide, and heatingthe solution to boiling point to precipitate out hydrated titaniumdioxide; f) washing the hydrolysate with an ammonium solution to removeresidual sulphate as ammonium sulphates followed by filtering off thehydrated titanium dioxide; or g) filtering the hydrolysate followed bywashing with sodium hydroxide, ammonium hydroxide, water, phosphoricacid and/or diluted sulphuric acid; and h) calcining the hydrolysate todrive off any residual acid and water of crystallisation to producetitanium dioxide.
 2. A method according to claim 1, wherein the slag instep a) is ground to form a particulate material in which at least 80%of the particles are able to pass through a 45 micron mesh.
 3. A methodaccording to claim 1 or claim 2, wherein the reaction of the sulphuricacid solution and particulate feed material in step b) takes place in afusion reactor.
 4. A method according to claim 3, wherein thetemperature is raised in the fusion reactor by introducing pre-heatedair into the reaction vessel.
 5. A method according to any one of claims1 to 4, wherein cold air is blown through the cake material after thereaction is completed to produce a porous cake.
 6. A method according toclaim 5, wherein the cold air is blown through the cake material forabout 4 hours.
 7. A method according to claim 5 or claim 6, wherein theporous cake is left to mature.
 8. A method according to claim 7, whereinthe porous cake is left to mature for about 8 hours.
 9. A methodaccording to any one of claims 1 to 8, wherein in step c) air isintroduced with the water, and optionally recovered process acid, inorder to assist with agitation to dissolve the cake.
 10. A methodaccording to claim 9, wherein the air is cold air to control thereaction temperature in order to prevent premature crystallisation ofTiO₂.
 11. A method according to claim 10, wherein the reactiontemperature is kept below about 85° C.
 12. A method according to claim11, wherein the reaction temperature is kept below about 75° C.
 13. Amethod according to any on of claims 9 to 12, wherein after the desiredamount of water has been introduced, air and mechanical agitation isused to break the cake into a homogenous suspension.
 14. A methodaccording to any one of claims 1 to 13, wherein the hydrolysis step e)is carried out in the absence of a prior crystallisation and vacuumconcentration step having taken place.
 15. A method according to claim1, wherein the titanium produced in step h) is further purified.
 16. Amethod according to claim 15, wherein the additional purificationcomprises the steps of: i) bricketing or pelletising the titaniumdioxide and subjecting it to a chlorination step in order to producegaseous TiCl₄; j) condensing the gaseous TiCl₄ to produce a crude liquidTiCl₄; k) distilling the crude liquid TiCl₄ to produce a substantiallypure TiCl₄ liquid, and either l) gasifying the pure TiCl₄ liquid toproduce gaseous TiCl₄ and oxidising the gaseous TiCl₄ to producetitanium dioxide pigment, or m) treating the pure TiCl₄ liquid in aconventional process to produce metallic titanium.